Historical Overviews of cases and influential factors:
Just as we discussed in part 1 of this thesis that the phenomenon of the DEF, which is the primary focus of this study that had to deal with the delayed formation of the mineral called ettringite which generates expansions in the concrete, deteriorated concrete and consequently leading to cracking mapped on the concrete surface, a weak pile cap and a weak wall.
The typology of cracking depends on the state of tension in which the element is subjected, as well as the density on the distribution of the reinforcement. Highlights of the ettringite is not systematically detrimental to the concrete since the same is the product of the cement hydration.
The first case assigned to the DEF as main cause factor of damage happened in 1987, in Finland, affecting concrete precast sleepers that had been subjected to an improper heat treatment and exposed to moisture. Beyond Finland, other countries in the world also reported this phenomenon in sleepers and in various different types of precast elements after about 10 years of operation (LCPC). Apart from precast components, the DEF was also responsible for damaging several bridges in Britain, ranging between 8 to 20 years after construction.
In France, initial cases were identified in the 1990s, between 5 and 10 years after construction, being that the concretes, mostly, were affected by DEF and not by alkali-silica reaction. In both cases, such concretes were more frequently casted during the summer, they had high cement content (between 420 and 550 kg/m3) and high alkali content equivalent (> 4.0 kg/m3). The tendency of these structural elements was to be quite thick (at least 60 cm), exposed to moisture and the maximum temperature reached inside these was estimated at approximately 80°C (LCPC) [7] . In addition to these, it was reported in southern Sri Lanka, the presence of severe cracks that affected some pile caps of the pillars of a highway bridge. After extensive investigations, it was verified that the main cause for the cracking in these pile caps was attributed to DEF (Nanayakkara [8].
In the north-american state of Maryland, Amde et al. [9] conducted a study in a large population of bridges that had mapped cracks. It was confirmed that almost a total presence of DEF in these cases contrary to the limited number of cases of alkali-silica reaction there until a few cases of coexistence between these pathologies.
In Brazil, there are rare reports of deterioration of concrete structures assigned to the DEF. Recently, an extensive investigation was performed to evaluate the probable causes that led to severe cracking in pile caps foundation of a building in the country. Based on the data for determination of the composition of the concrete and thereof, the approximate cement content, the possibility of the concrete mass had reached 80°C which was immensed. Most likely the concretes experiments in association with the AAR, a characteristic attack of DEF (HASPARYK et al.
According to Mehta and Monteiro [10], there is general consensus among researchers that the DEF occurs when the source of sulphate ions is internal rather than external, arising from the use of an aggregate contaminated with gypsum or cement containing high sulfate content in the production of the concrete. Additionally, steam curing of concrete parts above 65°C can induce the delayed ettringite formation, because the same is not stable above 65°C, decomposing to form hydrated monosulfate, which is adsorbed by the CSH. Subsequently, the sulphate ions are dissolved and originate the DEF causing expansion and cracking. According to Godard and Divet [11], when the maximum temperature in the concrete is greater than 65°C, derived from the thermal treatment or of the heat of hydration, the sulphates can be incorporated in other cement phases. The DEF is then defined as the ettringite formation in the concrete after hardening and without any external supply of sulfate, but exposed to moisture. This problem of the DEF may occur in thick concrete elements as crossbeams, bridge pillars and foundations where the core temperature may be too high, as a result of the heat of hydration of the cement (Nanayakkara [8]).
The LCPC [7] refers to the DEF, exclusively, to the concretes that were exposed at early age to the heating which exceed 65°C. Above this temperature, the primary ettringite formed during cement hydration reactions decomposes and generates an internal source of sulphate ions. After returning to ambient temperature and in the presence of moisture, the ettringite is able of was to recrystallize in a stage in which the concrete is hardened, generating swelling pressures which cause cracking. The maximum temperature reached and the application time of high temperature considerably influence the risk of DEF.
However, Collepardi [12] proposed a holistic model for DEF and indicates that it should include two distinct types: the DEF caused by internal sources of sulfate and DEF caused by external sources of sulfate. This also comments that the thermal decomposition of the primary ettringite and the sorption-desorption of sulfate by the CSH are not sufficient conditions and essential for DEF.
There is no consensus among researchers on the DEF mechanism and the cause of the expansion, but all agree that the expansion due to DEF occurs in concretes submitted to high temperature at early age. Therefore, Nanayakkara [8]and Escadeillas et al. [13] cite that was proposed at the International Workshop RILEM TC 186 - ISA that the DEF should be correctly known as "Heat Induced Internal Sulphate Attack".
Therefore, in this study, the term "DEF" will only reference to the delayed ettringite formation caused by internal attack sulfates induced by the Portland cement hydration heat.
In chemical composition, Collepardi et al. [14] cites that relatively high contents of SO3, much in the clinker (2%), as in the cement (4%), by adding gypsum, may aggravate the danger of causing high expansions related to the DEF when under heating of around 80-90°C. The LCPC [7] mentions that the DEF can only arise if the cement used contains a high content of C3A and sulfuric anhydride (SO3). Regarding the alkali content, this plays a fundamental role in the progress of the DEF, being that the ettringite is much more soluble at higher rates of alkali metals.
Leklou et al. [15] comment that the development of DEF is possible even in cements with low C3A content (<5%). However, for cements with high levels of C3A and SO3, the alkali of content is essential and a small variation in its content can allow or prevent the appearence of the DEF. In his work, Escadeillas et al. [13] confirm that high levels of alkalis increment the expansion risk of concrete after thermal curing.
Stark and Bollmann [16] stated that the DEF is the result of complex processes of long duration, where the concrete composition, technological factors of production and environmental effects on the concrete are important. Therefore, all these factors must be taken into consideration, since it is not enough consider only one of the factors that influence their triggering.
The cement fineness certainly also has an important role in DEF process. Generally, high early strength cements are particularly susceptible to expansion induced by DEF (CIGROVSKI [17]). In your experience in the limitation of temperatures reached by the concretes in several projects, aiming to prevent the DEF, Cussigh [18] shows that in general, the internal cooling is not needed in face of the optimization of the proportioning and of the component materials of the concrete.
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